This invention relates to drilling and cementing slim hole boreholes.
Boreholes for applications such as oil wells are normally drilled with an initial diameter of about 20 inches. It has long been known to drill smaller diameter holes, known as slim holes, for exploratory wells. These wells generally have a borehole of no more than 5-6 inches initial diameter. It would be desirable to drill slim hole production wells in some instances.
As with the drilling of conventional boreholes, the slim hole process is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill string while rotating to create a borehole into the earth. The drill string is hollow and sections are added to drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole. As this done, a layer of solids is deposited on the borehole wall which is commonly referred to as filter cake. The filter cake is formed by the combination of solids in the drilling fluid and the differential between the fluid pressure in the borehole and the formations being penetrated. Since the pressure exerted by the fluid column in the borehole is preferably slightly to significantly higher than the pressure in the pores of the exposed formation, there is a tendency for the liquid phase of the drilling fluid to leak off into the formation. As this occurs, the solids are deposited along the borehole wall since they are typically of sufficient size to prevent substantial penetration into the formation.
Once the borehole has been drilled to the desired depth, it may be desirable to isolate the separate areas, zones or formations transversed by the borehole. For extraction of fluids from formations, a conduit (casing) must be inserted into the borehole extending from the surface downward.
At this point it becomes necessary to fill the annulus between the casing and the borehole wall with a material which will seal the annulus and provide structural support for the casing. This is commonly referred to as primary cementing.
Slim hole drilling creates two problems. First, it is more difficult to remove the fluid in the annulus between the borehole and casing because the annulus is smaller. Second, it is more difficult to remove the filter cake on the side of the borehole wall, again because the annulus is so narrow. This latter problem is significant because the filter cake is generally incompatible with the cement. This can result in channeling of fluids used to wash out the drilling fluid and/or channeling of the cement leaving significant areas of unremoved and incompatible drilling fluid in the annulus. This results in voids in the final cementing job.
It has long been known to use pozzolans which broadly encompass, inter alia, slag as cementitious materials, as shown in Tragesser, U.S. Pat. No. 3,557,876 (Jan. 26, 1971).